1. Field of the Invention
The present invention relates to a susceptor for semiconductor manufacturing equipment which retains a semiconductor raw material by means of electrostatic charge, and more particularly, a susceptor for semiconductor manufacturing equipment formed by laminating plural aluminum nitride ceramic substrates with a high melting point metallic layer and an adhesive layer, the susceptor being used for a surface treatment of a silicon semiconductor wafer material.
2. Description of the Background Art
In the production of an LSI, an integrated circuit having fine wiring patterns is formed on the surface of a silicon semiconductor wafer. In order to establish an electrical insulation between the wiring patterns, an insulating film of silicon dioxide, silicon nitride or the like is formed by using various CVD means, such as plasma CVD, normal pressure CVD, etc. Hereinafter, this is referred to as xe2x80x9csurface treatmentxe2x80x9d. In this surface treatment, the wafer is treated one by one and a susceptor for retaining the wafer is required. This will be explained about the case of an electrostatic chuck method using a ceramic susceptor (chuck) by way of example. As schematically shown in FIG. 1, a semiconductor wafer 2 is placed on the susceptor 1. Reference numeral 9 denotes a thick-film electrode for endowing an electrostatic chucking function by applying an electric field to the susceptor through a direct current power source 10. The electrode is generally composed a metallic layer of a high melting point metal such as W, Mo, etc. In general, a heater 3 for heating up the wafer 2 is provided under the susceptor. A direct current power source 7 gives an electric field to an electrode 6 and a CVD material gas 4 is supplied in a vacuum chamber 5 from an upside of the susceptor 1, and plasma 8 is electrically generated. In such a manner, a film as mentioned above is formed on the surface of the wafer 2. The susceptor is required to have a high thermal conductivity, so as to rapidly conduct heat from the heater, so that the wafer uniformly heated. Further, in order to retain the retaining position of the wafer with a high precision, the susceptor is required to have a high dimensional precision.
A fairly high direct current voltage (generally about 1 kV) must be applied between the electrode for chucking and the ground. For this, the aforesaid thick-film electrode is formed. A method for this film formation includes the so-called co-fire metallizing method and the so-called post-fire metallizing method. The former is a method, in which a layer of a material comprising mainly the high melting point metal is formed (generally by printing and coating a conductive paste) on a green body (green sheet) of ceramic powder, followed by laminating, and the ceramics and the high melting point metallic layer are simultaneously sintered. The later is a method, in which a layer of a raw material comprising mainly a high melting point metal is formed (generally by printing and coating a conductive paste) on a substrate composed of a sintered ceramic body and then several these sintered bodies are laminated. Further, the high melting point metallic layer is fired by sintering. The former is advantageous in producing a multilayer wiring substrate for a semiconductor device at a low cost as compared with the latter. However, in the former method, since a large number of thin sheets are laminated, the resultant sintered bodies are subjected to a large deformation. Therefore, this method is undesirable when a susceptor having a large outer diameter and a high dimensional precision is desired.
The outer diameter of a wafer to be surface-treated is rapidly becoming large. Therefore, the size of a susceptor for holding it is also required to be large and, in the case of a circular shaped susceptor, the currently required size is 200 to 300 mm in diameter and its thickness is generally about 5 to 50 mm. However, it is considered that a susceptor having a larger diameter will be demanded in the future. Also, with the increasing trend toward fine and highly dense wiring on wafers, it will be increasingly important to ensure a high precision in the arrangement of wiring patterns. Therefore, the precision of the retaining position of the wafer should be highly improved, and the dispersion of the thickness of the susceptor in the direction of the main plane and the demanded tolerance of the flatness are becoming severe. For example, there is a severe demand in that when the outer diameter is 200 mm and the thickness of 5 mm, the allowable tolerance of the thickness is plus/minus several tens xcexcm and that of the flatness is about 100 xcexcm. As described above, there is a strong demand for larger suceptors with a higher dimensional precision. Therefore, there are difficulties in applying conventional production processes of multilayer wiring substrates for semiconductor devices without modifications.
The mode for retaining (chucking) a wafer includes a mechanical method chucking mechanically, an electrostatic chucking method using electrostatic charge, and a vacuum suction method sucking with vacuum. Among them the mechanical method has been mainly used in which an aluminum (Al) article that has an enhanced corrosion resistance to plasma by forming an anodic oxide film is used as a susceptor. However, in recent years the spacing between wirings is narrower with rapid increase in density of an LSI. When the wafer or susceptor is corroded by plasma, they generate dusts. If the dusts are adhered on the wafer as contaminants, there is a serious problem in that the wiring patterns of an LSI is broken and forms short circuit, to decrease the yield of the wafer. In order to avoid this problem, it is required to use a susceptor having a superior corrosion resistance to a raw material gas or often clean a chamber. Cleaning means removing the dusts by using a gas, such as NF3, CF4, etc., which has a higher corrosion activity than the raw material gas and reacts with the dusts and such a gas is hereinafter referred as xe2x80x9creaction mediumxe2x80x9d or simply as xe2x80x9cmediumxe2x80x9d. The above-mentioned susceptor made of aluminum is liable to be corroded by the medium. Accordingly, in order to solve the problems, an electrostatic chuck method using ceramics having excellent corrosion resistance as a susceptor has been used in recent years, as described in Ceramics, vol. 30, No. 11, p. 999 to 1001 (hereinafter, the ceramic susceptor used in this method is called xe2x80x9can electrostatic chuckxe2x80x9d or simply called xe2x80x9ca chuckxe2x80x9d).
As described in the foregoing article, a material suitable as such a electrostatic chuck includes ceramics composed mainly of alumina (Al2O3), aluminum nitride (AlN), boron nitride (BN), etc. Among these, aluminum nitride-based ceramics comprising mainly aluminum nitride (AlN) (hereinafter simply called xe2x80x9caluminum nitridexe2x80x9d or xe2x80x9cAlNxe2x80x9d) is excellent in corrosion resistance to the medium and also is excellent in thermal conductivity. When aluminum nitride ceramics is sufficiently densified, for example, defects such as pores are extremely small to have a density of 98% of the theoretical density, it has a high corrosion resistance to a fluorine compound and the amount of dusts generated from the chuck can be largely decreased. Accordingly, the contamination of the wafer described above can also be prevented, and simultaneously the life of the chuck itself can be prolonged. Due to its high thermal conductivity, even when the outer diameter of a wafer becomes large, uniform heating of the wafer is relatively rapidly conducted owing to its quick uniform heating. In order to form a circuit having fine patterns with uniform quality and thickness, it is necessary to precisely control the surface temperature of a wafer. Accordingly, the electrostatic chuck made of aluminum nitride ceramics is being spread quickly.
A method for the preparation of such a ceramic substrate includes a method in which thin molded bodies (green sheets) are laminated and sintered and a method in which relatively thick bodies (compacts) are sintered. For example, in order to obtain an electrostatic chuck having a thickness of 5 mm by the former method, 10 sheets or more of thin green sheets having a thickness of approximately 0.5 mm are prepared and laminated. In this case, a large amount of an organic binder is required to be incorporated into the thin sheets in order to retain the shape thereof. Therefore, the proportion of volatile components which are driven off during the firing step is increased and the sintering shrinkage becomes considerably large. For example, in the case of aluminum nitride which per se shows a relatively large shrinkage on sintering, when it is sintered in a such manner, considerable warp or deformation cannot be avoidably occurs and thereby adhesion among the individual lamination units becomes insufficient. As a result, the lamination interface of the sintered laminate is liable to be peeled (hereinafter, also referred to as xe2x80x9cvoidxe2x80x9d). The dispersion of shrinking percentage in the main plane is generally about 1%. However, if the deformation is large, a finishing treatment for a long period of time in the main plane is required in order to meet the severe requirement for high dimensional precision. Further, the dispersion in thickness in the direction of the main plane of the finished body becomes large. Furthermore, thick-film electrodes (shown at 9 in FIG. 1) interposed between ceramic substrate layers will be deformed corresponding to the deformation of ceramics. For example, the electrodes are formed with a pattern as shown in FIG. 3 in the direction of the main plane. When the dimensional dispersion of the ceramic substrate is large, the dimensional dispersion of the electrode pattern also becomes large. Due to such dispersion in thickness of the substrate or electrode pattern, unevenness of the adhesion performance on chucking (i.e., chucking adhesion power) is formed in the main plane. Therefore, when using sheets prepared by the doctor blade method, the outer diameter that is stably produced is 100 mm at most. Accordingly, when it is applied to one having a large diameter having an outer diameter exceeding 100 mm, which is being demanded particularly in recent years, the yield of products is poor, and the mass productivity is poor. In order to improve this, the latter method, in which the proportion of the volatile component during firing is relatively small, is advantageous as compared with the former method.
On the other hand, the latter method is a method in which material powders are molded under pressure after being filled in a mold (dry molding) or molded by extruding a kneaded article thereof, followed by sintering. In this method, sintering procedures of the ceramics main body and the high melting point metallic layer are conducted separately, and a product having a relatively large thickness can be molded. Accordingly, the problems in the former can be relatively easily avoided. When polish finishing of both the main planes after sintering, straightening at a high temperature and simultaneous sintering bonding on application of heat and pressure are conducted, substantially no deformation occurs on the subsequent firing of the high melting point metallic layer. Therefore, a product having higher dimensional precision than the former method can be sufficiently obtained.
However, since aluminum nitride ceramics is difficult to be sintered when its constitutional component is only AlN, generally a sintering aid containing a Group 2a element (Be Mg, Ca, Sr, Ba and Ra) or a Group 3a element (Y, Sc, lanthanum series elements and actinium series elements) is added to aluminum nitride and sintering is conducted at a high temperature of 1,600xc2x0 C. or more under a liquid phase of the sintering aid. While these sintering aids generally become vitreous materials, these vitreous materials are liable to be decomposed by the above-mentioned high temperature plasma containing fluorine. The reaction product formed by decomposition of the vitreous materials is volatilized and dispersed into the high temperature medium to become dusts, and is released from the susceptor to remain as a hole. This lowers the corrosion resistance of the susceptor and shortens the life of the susceptor itself. Further, dusts are newly generated from the part of the holes and cause contamination of wafers. It is therefore preferred that the amount of the sintering aid to be added be small. However, when the addition amount of the sintering aid is too small, the sintering becomes difficult to cause a problem in that a dense product cannot be obtained.
Japanese Patent Publication No. 63435/1993 discloses aluminum nitride ceramics in which a Group 3a element as a sintering aid is uniformly dispersed in a small amount and contains a small amount of a vitreous phase. In column 7, lines 26 to 34 of the publication, there is suggested that because the aluminum nitride ceramics contains a small amount of a vitreous phase composed of the sintering aid, it is relatively suitable as the usage aimed in the invention.
The inventors have continued studies and investigations based on the material described in the above publication to provide an electrostatic chuck that could be stably produced with maintaining the above-mentioned dimensional precision and practical performance. As a result, an electrostatic chuck of high quality that has not been obtained conventionally could be obtained by modifying or specifying the composition and the method for molding and sintering while controlling the AlN crystal particle size or the amount of defects of the aluminum nitride ceramics so as to ensure a high dimensional precision.
Accordingly, the electrostatic chuck that the invention provides is (1) a susceptor for semiconductor manufacturing equipment formed by laminating plural substrates comprising an aluminum nitride ceramic with a high melting point metallic layer and an adhesive layer. In the electrostatic chuck of the invention, (2) it is preferred that the aluminum nitride ceramic comprises a compound of a Group 3a element in an amount of from 0.01 to 1% by weight in terms of the element, and the average particle size of an AlN crystal is from 2 to 5 xcexcm. It is preferred that (3) the thermal conductivity of the aluminum nitride ceramic is 150 W/mxc2x7K or more.
Further, the invention includes (4) a susceptor in which the uppermost substrate of the laminated substrates comprises a ceramic other than aluminum nitride ceramic and (5) a susceptor in which the uppermost substrate of the laminated substrates is coated with a diamond layer. Furthermore, the invention includes (6) a susceptor in which the substrate has 5 or less of pores having a maximum diameter exceeding 1 xcexcm at a triple point of grain boundaries in an arbitrary rupture section of 1,000 xcexcm2 and (7) a susceptor in which the high melting point metallic layer comprises at least one element selected from the group consisting of W, Mo and Ta, and in which the same layer comprises low melting point glass. is preferred that (8) the low melting point glass is an oxide glass comprising at least one element selected from the group consisting of Ca, Al and Si, and the invention involves (9) a susceptor in which the adhesive layer comprises 80% by weight or more of aluminum nitride (AlN) and the balance consisting essentially of a compound of an element of Group 2a and an element of Group 3a of the periodic table.
The production process of an electrostatic chuck provided by the present invention is as follows: That is, the invention provides (1) a process comprising a step of forming a mixture by adding a powder of a sintering aid to a powder of aluminum nitride and mixing these powders (step 1); a step of forming a molded body by molding the mixture (step 2); a step of forming a sintered body by firing the molded body in a non-oxidizing atmosphere at a temperature range of from 1,600 to 2,000xc2x0 C. (step 3); a step of forming a substrate by working the sintered body into a desired shape (step 4); a step of preparing a plurality of the substrates, and a material forming a high melting point metallic layer and a material forming an adhesive layer as insertion materials (step 5); a step of forming an assembly by laminating the substrates with sandwiching the insertion materials (step 6); a step of firing the assembly into a laminate in a non-oxidizing atmosphere at a temperature range of from 1,500 to 1,700xc2x0 C. (step 7); and a step of finishing the laminate (step 8).
The present invention includes (2) a process in which in the step 1, a powder of a compound containing at least one element selected from elements of Group 3a of the periodic table in an amount of from 0.01 to 1% by weight in terms of the element is added and mixed as the sintering aid; (3) a process in which in the step 3, the molded body is fired in a non-oxidizing atmosphere at a temperature range of from 1,600 to 2,000xc2x0 C., and then cooled to 1,500xc2x0 C. at a cooling rage of 200xc2x0 C. per hour or more to form a sintered body; (4) a process in which in the step 5, a ceramic other than an aluminum nitride ceramic is provided as the uppermost one of the substrates to be laminated; and (5) a process in which the step 8 includes forming a diamond layer onto the uppermost substrate of the laminated substrates.